Field of the Invention
The present invention relates to a measurement apparatus, a lithography apparatus, and an article manufacturing method.
Description of the Related Art
Recently, exposure apparatuses have undergone progress of refinement of exposure accuracy, and have been strictly required for accuracy in positioning at the same time. In measurement for positioning, an image of a mark placed on a reticle that is an original is fetched by a measurement device for a mark position, such as a CCD camera, and the mark position is measured by processing the fetched image signals with a measurement apparatus. Hence, it is optimum to keep the measurement device for measuring the mark position and the mark in a state free from minute fluctuation due to vibrations or the like during performing the measurement for the positioning.
If the stage where the mark is placed and the measurement device for the mark position are not supported by an identical housing, it is difficult to eliminate relative fluctuation between the stage and the measurement device due to vibrations of a floor. Consequently, the relative fluctuation between the stage and the measurement device during a time period of fetching the image signals causes deterioration of the measurement accuracy.
As one solution for such problems, Japanese Patent No. 3548428 discloses a method of finding an average position of the mark during a time period of fetching image signals, for example. An apparatus described in Japanese Patent No. 3548428 includes a measurement device to measure a position of the stage several times, and averages positional data measured several times, thereby enhancing measurement accuracy.
The positional measurement apparatus disclosed in Japanese Patent No. 3548428 finds an average position of the mark based on the accumulated image signals. In the case of having a shorter relative fluctuation period between the stage and the measurement device than a time period of fetching the image, a fluctuation component between the stage and the measurement device can be offset by finding the average position of the mark, thereby enhancing the measurement accuracy of the mark position. To the contrary, in the case of having a longer relative fluctuation period between the stage and the measurement device than the time period of fetching the image, a fluctuation component between the stage and the measurement device cannot be offset even if the average position of the mark is found; thus the measurement accuracy of the mark position is not enhanced. It may be considered to increase a frequency of the measurement of the stage position in accordance with the relative fluctuation period between the stage and the measurement device, which makes the measurement time period longer, resulting in deterioration of productivity in the apparatus.